In the field of electronic circuit assembly, electronic components are often carried from a supply of such components to a specific location on a circuit board for attachment thereto. The components may be of several different types, including surface mount components and through-hole components. Specifically, these components may include, but are not limited to such things as memory chips, resistors, connectors, dual in-line processors (DIPS), capacitors and gate arrays. These and other components are typically affixed to a circuit board that may later be incorporated into an electronic device.
Rather than manually affixing each individual electronic component to a circuit board, the electronics industry makes extensive use of robotic placement machines, sometimes known as "pick-and-place" machines, which grasp a component at a specific location (the supply) and place it at another specific location (the circuit board). To ensure the sustained operation of such robotic placement machines, a continuous supply of electronic components must be furnished to the machine at a predetermined location in order that the robotic placement machine may be programmed to repeat a precise sequence of movements during every cycle. It is therefore important that each such component be located in the same position (i.e. the point at which the robotic placement machine grasps the component) as each preceding and succeeding component.
One way to provide a continuous supply of electronic components to a desired location is to load a series of such components into pockets that are spaced along a carrier tape. The loaded carrier tape, which is usually provided in roll form, may then be advanced toward the pick-up location at a predetermined rate as each succeeding component is removed from the tape by the robotic placement machine. Conventional carrier tapes generally comprise a self-supporting lower portion that carries the component, and a flexible cover strip that aids in preventing foreign matter from deleteriously affecting the component. The cover strip is typically bonded to the lower portion, and is progressively peeled away from the lower portion just before the robotic placement machine removes the component from the carrier tape. Alternately, a rigid reusable cover strip may be used, such as that described in U.S. Pat. Nos. 4,733,778, 4,842,135, 4,844,258, 4,958,053, and 5,005,275.
FIG. 1 illustrates a conventional carrier tape 10 in combination with a robotic placement machine 12. A supply roll 14 provides carrier tape 10 having an aligned plurality of regularly spaced pockets 16, each of which is loaded with an electronic component. A stripper assembly 18 peels the cover strip 20 from carrier tape 10 around a stripper block 22, which assists in preventing stripper assembly 18 from pulling carrier tape 10 away from its designated path. Carrier tape 10 is advanced by a sprocket 24, which engages advancement holes along at least one outer edge surface of carrier tape 10 to advance carrier tape toward robotic placement machine 12. As each successive component reaches the desired pick-up point, the robotic placement machine grasps the component (either manually or by suction) and places it on a circuit board in the appropriate location.
Electronic components may be formed by encapsulating an integrated circuit within a plastic compound. However, if the component contains entrained moisture, cracking can occur when the component is later heated for soldering to a circuit board. It is therefore desirable to remove as much entrained moisture as possible from the component in order to minimize the possibility of cracking. Moisture removal is usually done by baking or drying the components in an oven to evaporate the entrained moisture prior to soldering the components to a circuit board. Baking and drying are synonymous as used herein. The baking conditions vary with the application, and are generally divided into low temperature baking and high temperature baking. See R. Prasad, Surface Mount Technology: Principles and Practice pp. 190-92 (1989), the entire contents of which is hereby incorporated by reference. Low temperature baking typically occurs at a temperature of approximately 40.degree. C. (.+-.5.degree.) at less than 5% relative humidity for a minimum period of 192 hours (8 days). Because of the extended time period required for effective low temperature baking, high temperature baking is generally preferred. High temperature baking typically occurs at a temperature of 125.degree. C. (.+-.5.degree.) at less than 50% The duration and temperature may vary from the illustrative ranges described above depending on the construction of the components, the amount of moisture present, and other operating conditions.
Components are presently baked in temperature resistant trays or tubes to remove excess moisture and then loaded into carrier tapes for storage and delivery. It would be desirable to dry the components within the carrier tape, to eliminate the intermediate processing step of loading and unloading the components from the baking trays or tubes for baking in the oven. However, carrier tapes constructed of conventional materials are generally not suitable for high temperature baking because the carrier tape and cover strip may begin to degrade at elevated temperatures. Degradation of the carrier tape may cause warping, shrinkage, or other undesirable effects that could prevent the carrier tape from being used effectively. For example, if the floor or walls of the pocket deform due to high baking temperatures, an electronic component may shift within the pocket and prevent the placement machine from grasping the component. Alternately, the cover strip could become pliable or change shape when it is heated, and could perhaps become adhered to the components within the pockets. The components may become damaged in an attempt to remove them from the cover strip, thereby rendering the components and carrier tape useless and resulting in a substantial loss for the packager.
Even if a heat-resistant carrier tape and cover strip are provided, another difficulty has been shown to render in situ baking undesirable. The adhesive that bonds the cover strip to the carrier tape tends to form a strong bond between the cover strip and the carrier tape when it is exposed to high temperatures. That is, the adhesive bonds the cover strip to the carrier tape so effectively that facile removal of the cover strip becomes difficult or impossible. For example, the cover strip may release only after the application of high peel forces, which can dislodge components that are in uncovered pockets. Alternatively, the high peel forces may cause the cover strip to tear, which could require the production line to halt in order to reattach the cover strip to the stripper assembly. Although the temperature of the baking step could be reduced in an attempt to prevent the overbonding problem, the time necessary to evaporate extant moisture correspondingly increases as described above, which slows the production process.
In view of the disadvantages of conventional carrier tapes, it is therefore desirable to provide a carrier tape and cover strip for electronic components that can withstand the temperatures encountered during the drying process and yet allow for facile removal of the cover strip to provide access to the contents of the pockets.